It is known that an electrically conductive composite material can be obtained by mixing an electrically conductive filler such as carbon black, carbon fiber, metal powder and the like in a resin material such as thermosetting resin, thermoplastic resin and the like.
However, in order to give high electric conductivity, particularly preferably volume resistivity of 1*106 ohm cm or less, to this type of composite materials, it is necessary to add a considerable amount of conductive filler. The conductive filler added in a large quantity gives a bad influence to the properties of the resin material, and original characteristics of the resin might not be exerted in the prepared composite material.
Thus, a filler is in demand which can, even with a small amount, give sufficient conductivity.
A vapor grown carbon fiber as the conductive filler includes those in a structure in which a graphite layer is inclined to the fiber axis which is referred as fishbone type or herringbone type, those in which a graphite layer is substantially at a right angle to the fiber axis which is referred as platelet type, no-hollow carbon fiber or carbon fiber with an obscure graphite layer. They are called “carbon nanofiber” in general.
On the other hand, there are known that a carbon fiber with the graphite layer is substantially in parallel with the fiber axis which is referred as tube type, and the carbon fiber with such a structure is called “carbon nanotube”.
PTL 1 discloses a fishbone-type carbonaceous fine-fibrous body that is carbon nanofiber with excellent kneading performance with a resin and good conductivity. According to PTL 1, it is considered that asperity formed by commissure interval between bone chips on a side peripheral portion of the fibrous body and planar interval between the bone chips give a good influence on conductivity and kneading performance. Moreover, it is disclosed that, as compared with the carbon fiber (carbon nanotube) with an angle (theta) of the graphite layer at substantially 0 degree that is parallel with the fiber axis, the fishbone type carbonaceous fine-fibrous body gives lower volume specific resistance when made into a resin composite material. However, the carbon nanofiber shown in PTL 1 has many residues from a catalyst component when it is calculated from the examples described in the gazette. If there are many impurities in the resin composite material, there is a problem that mechanical properties of the composite material such as strength and the like are extremely lowered because of invitation in decomposition of the resin by the impurities in molding and the like.
As a method for producing carbon nanofiber or carbon nanotube, a method for growing a carbon fiber using a catalyst as a core, namely, a so-called chemical vapor deposition method (hereinafter referred to as CVD method) is known. As the CVD method, a method in which a catalyst metal is supported on a carrier for use and a method in which an organic metal complex or the like is thermally decomposed in a vapor phase so as to generate a catalyst without using a carrier (fluidized vapor phase method) are known.
As the carbon nanofiber obtained by the method of generating a catalyst in a vapor phase (fluidized vapor phase method), PTL 5 shows a carbon fiber having a total metal element content of 0.3 to 0.7% by mass and a transition metal element content of 0.1 to 0.2% by mass obtained by a fluidized vapor phase method in which an organic metal complex such as ferrocene and a carbon source such as benzene are fluidized, and the carbon source is thermally decomposed under a hydrogen atmosphere using a metal particle as a catalyst obtained by thermal decomposition of the metal complex. However, since the fluidized vapor phase method requires a high-temperature reaction field in general, a manufacturing cost is raised. Since the carbon fiber obtained by the fluidized vapor phase method has many defects in a graphite layer and has a problem that without heat treatment at a high temperature for graphitization after the generation of the carbon nanofiber, electric conductivity does not emerge even if being added to a resin or the like as filler. Thus, with the fluidized vapor phase method, it is difficult to inexpensively produce a carbon nanofiber having desired properties.
On the other hand, a method using a catalyst carrier is roughly divided into (1) a method using a platy substrate carrier; and (2) a method of using a particulate carrier. With the method (1) using a platy substrate carrier, since the size of the catalytic metal to be supported can be arbitrarily controlled by applying various film formation technologies, this method is usually used in laboratory demonstration of research. For example, NPL 1 discloses that using those in which an aluminum layer having thickness of 10 nm, an iron layer having thickness of 1 nm, and a molybdenum layer having thickness of 0.2 nm are generated on a silicon substrate can give a tube-like multiwall nanotube or a double-wall nanotube having a fiber diameter of approximately 10 to 20 nm. In order to use the carbon nanotube as filler obtained by this method to be added in a resin or the like, it is necessary to separate it from the substrate and collect it. The carbon nanotube collected as the above substantially contains only catalytic metal component as impurities, but since generation efficiency of the carbon nanotube with respect to a catalyst mass is markedly low, the content of the catalytic metal component in the carbon nanotube is likely to be high. Moreover, if this method is to be industrially utilized, since a platy substrate surface area can not be ensured unless a number of substrates are arranged, not only that device efficiency is low but also many processes such as supporting of the catalytic metal on the substrate, synthesis of the carbon nanotube, collection of the carbon nanotube from the substrate and the like are needed, which is not economical, and industrial utilization has not been realized yet.
On the other hand, with the method (2) using the particulate carrier, as compared with the method using the substrate carrier, since a specific surface area of the catalyst carrier is larger, not only that the device efficiency is favorable but also a reactor used for various chemical synthesis can be applied, and this method has merits that realizes not only a production method based on batch processing such as the substrate method but also continuous reactions. However, with this method, a catalyst carrier is un-avoidably mixed in a carbon nanofiber or a carbon nanotube, and it is difficult to obtain a carbon nanofiber or a carbon nanotube with high purity.
As a method for reducing the amount of impurities in the carbon nanofiber or the carbon nanotube, (1) a method of heat treatment at a high temperature; and (2) a method of washing and removing with acid or base are known, but both of the methods have complicated processes and are not economical. Particularly, in the washing and removing of the impurities with acid or base, since the catalyst carrier and the catalytic metal in the carbon nanofiber or the carbon nanotube are covered by a carbon overcoat in many cases, it is difficult to fully remove the impurities unless the carbon overcoat is removed by using an oxidizing acid such as nitric acid or by performing partial oxidization. If an oxidizing acid is used, not only the carbon overcoat on the surface of the carrier or the catalyst but also the carbon nanotube itself might be damaged and might become defective. The carbon fiber affected by an acid might have lowered electric conductivity or lowered heat conductivity, or dispersibility or filling performance into a resin or the like might be deteriorated.
PTL 3 discloses a catalyst obtained by coprecipitation of a metal having fibril-forming catalytic properties composed of Fe or a combination of Fe and Mo and a carrier metal component such as Al, Mg and the like. It is disclosed that using this catalyst, a carbon fiber having the content of impurities from the catalytic metal of 1.1% by mass or less and the content of the impurities from the catalyst carrier of 5% by mass or less can be obtained. However, catalyst manufacture by coprecipitation is known to have low efficiency and high cost.
PTL 2 discloses a catalyst containing Fe element and at least one element selected from the group consisting of V, Nb, Ta, Cr, Mo, W, Mn, Tc, and Re. Specifically, PTL 2 discloses that the catalyst is obtained by supporting a metal element composed of a combination of Fe and Mo, Fe and Cr, Fe and Ce, Fe and Mn or the like on a carrier using the impregnating method. However, since the obtained carbon fiber has a high impurity content, when it is used as filler to a resin, there is a problem that strength of the resin is lowered.
PTL 4 discloses a supported catalyst obtained by coprecipitation of a catalytic metal component composed of a combination of Mn, Co, and Mo or a combination of Mn and Co and a carrier metal component such as Al, Mg and the like.